Pollen-coated sticky bullets track a gunman's DNA

FLOWERS and guns might conjure up images of flower power, but coating bullet casings in lily pollen could help forensic teams identify a gunman.

It is difficult to get useful DNA evidence from a spent bullet casing: copper and zinc ions from the brass alloy react with sweat to break down DNA, destroying evidence about who may have loaded a gun.

To combat this, Paul Sermon, a nanomaterials engineer at Brunel University in London, is leading a government-funded team to develop forensic coatings for brass bullet cartridges. The initial idea was to coat a bullet with a biochemical that stuck to the hands of those who touched it, allowing police to test the hands of suspects. Then they hit on a technique that could also stash away skin cells from that person. "We've combined these to increase the probability of obtaining useful associative evidence," says Sermon.

The challenge they faced was twofold: the coatings had to be compatible with the way bullets are made and cope with the heat generated when a bullet is fired. After years of experiments they say they have hit on a promising coating - and a way to apply it. "It's as simple as dunking a biscuit in a cup of tea," says Sermon.

Their first trick is to roughen the surface of the cartridge by dipping it in a solution of aluminium oxide and urea. When dry, this leaves an abrasive, ridged surface on which far more skin cells can be captured, increasing the chances that usable DNA can be recovered. Tested on bullets from a 9 millimetre Browning pistol, the team found that 53 per cent more viable DNA could be harvested from these bullets than from normal ones (Forensic Science International, DOI: 10.1016/j.forsciint.2012.04.021).

To label the hands of anyone who touches the bullet, they took the sticky pollen grains from the Easter lily, Lilium longiflorum, and coated them in titanium dioxide before dropping them in liquid plastic. This solution was used to coat the bottom of the bullet casing. While the pollen is not uncommon, and TiO2 is found in paints and sun lotions, together they form a unique tag, says Sermon.

"It's a fascinating development we'll watch with interest," says Mike Sweeney of BAE Systems, which makes ammunition for the British army.

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Who loaded the gun? (Image: West Coast Surfer/Mood Board/Rex Features)